The present invention relates to an intermediate transfer unit used in an image formation apparatus using an electrophotographic method, such as a copying machine, a printer, and a facsimile. The present invention also relates to a recording medium carrier system applied to the image formation apparatus.
As for a copying machine, a printer, a facsimile and other image formation apparatuses respectively using electrophotography, primarily an image formation apparatus using a laser beam writing device, it is important to fix a toner image while carrying a recording medium at high speed in order to make good use of the apparatus. It is also important to provide a simple means for relieving a paper jam or other problems caused by such operation.
Generally, an image formation apparatus using electrophotographic technology is provided with a photoconductive drum having a photosensitive layer as the peripheral face, charge means for evenly charging the peripheral surface of the photoconductive drum, exposure means for selectively exposing the evenly charged peripheral surface to form an electrostatic latent image, developing means for applying toner as a developer to the electrostatic latent image formed by the exposure means to form a visible image (a toner image), and transfer means for transferring the toner image developed by the developing means onto a transfer medium such as paper.
For transfer means for transferring a toner image developed on a photoconductive drum on a transfer medium, such as paper, heretofore, there is known transfer means provided with an intermediate transfer belt to which a toner image formed on a photoconductive drum is transferred (primary transfer) and which further transfers (secondary transfer) the toner image onto a recording medium, and with a driving roller for circulating the intermediate transfer belt.
As for the above prior transfer means, there is a problem that since a distance between a primary transfer position and the driving roller is large, the amount of shrinkage of the intermediate transfer belt between the primary transfer position and the driving roller is increased and the travel speed of the intermediate transfer belt in the primary transfer position is unstable. As a result, it is difficult to acquire satisfactory primary transfer.
Further, according to the above prior transfer means, there is a problem that a transfer roller directly touches the joint of the intermediate transfer belt, staining a secondary transfer roller by toner accumulated in a step of the joint of the intermediate transfer belt, and causing toner to adhere to the rear of a recording medium in a subsequent secondary transfer.
Further, according to the above prior transfer means, there is a problem that when a thin line image is transferred onto a recording medium, the surface of which is smooth, a failure of the transfer of toner (a void) occurs.
Further, according to the above prior transfer means, there is a problem that even if transfer on a recording medium having a smooth surface is satisfactory, transfer on a recording medium having a rough surface is insufficient. Particularly, when multiple layers of toner are transferred as a multiple color image, a failure to transfer toner far from the surface of a recording medium occurs.
Further, according to the above prior transfer means, there is a problem that in primary or secondary transfer, the deterioration of transfer efficiency and the omission (void) of a part of a toner image in transfer occurs. Also, in secondary transfer, there is a problem that it is difficult to transfer on a recording medium the surface of which is extremely irregular, such as recycled paper and bond paper, without lacking a part of an image. There is also a problem that if toner having a high fluidity is used, toner is readily scattered in transfer. In particular, if a primary or secondary transfer means which functions as a transfer electrode for applying transfer voltage to a transfer position, is located in a position distant from its transfer position, a transfer electric field in the transfer position cannot be concentrated upon the transfer position, and a toner image is scattered due to electrostatic force. For example, if the intermediate transfer belt is wound on the photoconductive drum without means for substantially pressing the intermediate transfer belt on the photoconductive drum or a recording medium in a transfer position, the area in which the photoconductive drum and the intermediate transfer belt are in contact in a transfer position is large and the turbulence of a toner image due to mechanical force caused by slight difference in speed between both and others readily occurs.
Further, according to the above prior transfer means, a monolayer or multilayer belt in which a conductive, a semiconductive or an insulating resin layer is generally formed, at least as the surface layer, is used for the intermediate transfer belt. Thus, there is a problem that, since the surface is made of resin as described above, friction and scratches are readily generated. In particular, a large quantity of particulates of metallic oxide generally adhere to the surface of a toner particle as an additive and, since the above additive is much harder than resin constituting the surface of the intermediate transfer belt, it is readily embedded in the intermediate transfer belt. Further, a phenomenon (so-called filming) in which toner adheres to the intermediate transfer belt in the embedded point, mentioned above, occurs and deteriorates the image. For example, the transfer efficiency in primary or secondary transfer deteriorates and a void (i.e., the lack of a part of a toner image in transfer) occurs. Also, in secondary transfer, there is a problem that it is difficult to transfer on a recording medium having a surface that is extremely irregular, such as recycled paper and bond paper, without causing an imperfection in an image.
Further, according to the above prior transfer means, there is a problem that a void occurs in a part of a toner image transferred on the intermediate transfer belt in primary transfer, particularly the center. Also, in secondary transfer, there is a problem that it is difficult to transfer on a recording medium having an extremely irregular surface, such as recycled paper and bond paper, without causing an imperfect image, in addition to the above problem of a void.
Further, in an image formation apparatus for forming a full color image by overlapping plural colors, for example, the secondary transfer means is prevented from being stained by controlling the driving of the secondary transfer means for executing secondary transfer so that the secondary transfer means is not in contact with the intermediate transfer belt while images of each color are formed. Instead, the secondary transfer means is touched to the intermediate transfer belt after the final image is formed and, when secondary transfer is started after primary transfer is finished, an image on the intermediate transfer belt is not disturbed. However, there is a problem that when the intermediate transfer belt is vibrated, such as when the secondary transfer means is switched to a state in contact or not in contact with the intermediate transfer belt, the speed is varied and turbulence of an image occurs.
Further, according to the above prior transfer means, transferability in a primary transfer part is insufficient. Concretely, there are problems in the quantity of toner (the thickness of the layer), dispersion in resistance among each member, the variation of transfer efficiency due to the variation of resistance, a phenomenon of a void, and the stability of the density due to aging.
Further, according to the above prior transfer means, transferability in a secondary transfer part is insufficient. Concretely, there are problems in, the quantity of toner (the thickness of the layer), the type of a recording medium such as plain paper, a postal card, and OHP sheet, dispersion in resistance and the variation of resistance among each member, the variation of transfer efficiency due to the variation of resistance by environment, a phenomenon of a void, and the stability of the density due to aging.
Further, with respect to resistance, which is an important characteristic of a primary transfer member and a secondary transfer member, the above transfer means includes members having approximately the same variation of resistance due to environment are used for both the primary and secondary transfer members. Therefore, if members having a small variation of resistance due to environment are used for both primary and secondary transfer members, current may leak in a part not related to transfer and the failure of transfer may occur, particularly in a case where a recording medium, such as a postal card or an envelope smaller in size than the width of the secondary transfer member, is printed in an environment of low temperature and low humidity in which the resistance of the recording medium is higher than that of the secondary transfer member in a secondary transfer part. To avoid the above situation, it is possible to increase the resistance of the secondary transfer member and reduce leakage current. However, since a member having small variation of resistance due to environment generally has a large dispersion of the resistance, there is a problem that the nonuniformity of transfer partly occurs.
In the meantime, if members having large variation of resistance due to environment is used for both primary and secondary transfer members, no failure due to a leak of secondary transfer occurs because the resistance of the secondary transfer member changes approximately as the change of the resistance of a recording medium due to environment. However, voltage required in a primary transfer part in the environment of low temperature and low humidity causes the cost to increase.
Further, in a prior transfer means as disclosed in Japanese Patent Application No. Hei. 7-322667, an imperfect image is prevented from occurring at the simultaneous timing of primary transfer and secondary transfer by providing a conductive layer on the intermediate transfer belt and setting a relationship between resistance R.sub.T of a part from a primary transfer bias applying power source to the conductive layer and apparent resistance R1 in a primary transfer part so that R.sub.T &lt;R1.
According to above prior transfer means, it is difficult, depending upon environment and the type of paper, to prevent an imperfect image from occurring at the simultaneous timing of primary transfer and secondary transfer. Concretely, if current which flows in a secondary transfer is larger than current which flows in a primary transfer, the phenomenon is remarkable.